1. Field of the Invention
This invention relates generally to a separation system to obtain a separation of particulate solids from a mixed phase gas-solids stream and particularly to a separation system for separating the spent catalyst from the cracked hydrocarbon effluent stream of an FCC riser reactor.
Chemical reaction systems utilizing solids in contact with a gaseous or vaporized stream have long been employed. The solids may participate in the reaction as catalyst, provide heat required for an endothermic reaction, or both. Alternatively, the solids may provide a heat sink in the case of an exothermic reaction. Fluidized bed reactors have substantial advantages, most notably an isothermal temperature profile. However, as residence time decreases the fluidized bed depth becomes shallower and increasingly unstable. For this reason, tubular reactors employing solid-gas contact in pneumatic flow have been used and with great success particularly in the catalytic cracking of hydrocarbons to produce gasolines where reactor residence times are between 0.5 and 5 seconds, optimally about 2 seconds.
In general, catalytic cracking of relatively high boiling hydrocarbons to form substantial quantities of materials boiling in the gasoline range is carried out in the following process sequence: hot regenerated catalyst is contacted with a hydrocarbon feed in a reaction zone under conditions suitable for cracking; the cracked hydrocarbons are separated from the spent catalyst using conventional cyclones and the spent catalyst is subsequently fed to a regeneration chamber where a controlled volume of air is introduced to burn the carbonaceous deposits from the catalyst after which the regenerated catalyst is transferred to the reactor for reuse.
With the advent of improved catalysts, total reactor residence times in some processes can be as low as 0.2 to 1.0 second. However, with residence times below 2 seconds and specifically below 1 second, the ability to separate the gaseous products from the solids is diminished because of the residence time requirements of conventional separation means such as cyclones. The residence time requirements of cyclones represents a disproportionate fraction of the allowable residence time. In an FCC system, conventional separation systems may consume more than 35% of the allowable contact time between the two phases resulting in product degradation, coke formation, low yields and varying severity. In catalytic cracking at lower or moderate temperatures, quench of the product gas in the presence of catalyst is undesirable from a process standpoint. In other processes, the quench is uneconomic in terminating the reaction in the presence of catalyst. Thus, these reaction systems require immediate separation of the phases to remove catalyst from the gas phase as a means for removing the reaction mechanism.
The prior art has attempted to separate the phases rapidly by use of centrifugal force or deflection means.
Nicholson U.S. Pat. No. 2,737,479 combines reaction and separation steps within a helically wound conduit containing a plurality of complete turns and having a plurality of gaseous product drawoffs on the inside surface of the conduit to separate solids from the gas phase by centrifgual force. Solids gravitate to the outer periphery of the conduit, while gases concentrate at the inner wall, and are removed at the draw-offs. Although the Nicholson reactor-separator separates the phases rapidly, it produces a series of gas product streams each at a different stage of feed conversion. This occurs because each product stream removed for the multiple product draw-offs which are spaced along the conduit is exposed to the reaction conditions for a different time period in a reaction device which has inherently poor contact between solids and gases.
Ross et al U.S. Pat. No. 2,878,891 attempted to overcome this defect by appending to a standard riser reactor a modification of Nicholson's separator. Ross's separator is comprised of a curvilinear conduit making separation through a 180.degree. to 240.degree. turn. Centrifugal force directs the heavier solids to the outside wall of the conduit allowing gases that accummulate at the inside wall to be withdrawn through a single drawoff. While the problem of product variation is decreased to some extent, other drawbacks of the Nicholson apparatus are not eliminated.
Both devices effect separation of gas from solids by changing the direction of the gas 90.degree. at the withdrawal point, while allowing solids to flow linearly to the separator outlet. Because solids do not undergo a directional change at the point of separation, substantial quantities of gas flow past the withdrawal point to the solids outlet. For this reason both devices require a conventional separator at the solids outlet to remove excess gas from the solid particles. Unfortunately, product gas removed in the conventional separator has remained in intimate contact with the solids, has not been quenched, and is, therefore, severly degraded.
Another drawback of these devices is the limitation on scale-up to commercial size. As conduit diameter increases the path traveled by the mixed phase stream increases proportionately so that large diameter units have separator residence times approaching those of conventional cyclones. Increasing velocity can reduce residence time, but as velocities exceed 60 to 75 ft./sec. erosion by particles impinging along the entire length of the curvilinear path becomes progressively worse. Reduction of the flow path length by decreasing the radius of curvature of the conduit also reduces residence time, but increases the angle of impact of solids against the wall thereby accelerating erosion.
Pappas U.S. Pat. No. 3,074,878 devised a low residence time separator using deflection means wherein the solid gas stream flowing in a tubular conduit impinges upon a deflector plate causing the solids, which have greater inertia, to be projected away from a laterally disposed gas withdrawal conduit located beneath said deflector plate. Again, solids do not change direction while the gas phase changes direction relative to the inlet stream by only 90.degree. resulting in inherently high entrainment of solids in the effluent gas. While baffles placed across the withdrawal conduit reduce entrainment, these baffles as well as the deflector plate are subject to very rapid erosion in severe operating conditions of high temperature and high velocity. Thus, many of the benefits of separators of the prior art are illusory because of limitations in their efficiency, operable range, and scale-up potential. Gartside et al U.S. Pat. Nos. 4,288,235, 4,348,364, 4,433,984 devised an apparatus for rapidly separating particulate solids from a mixed phase solids-gas stream from tubular type reactors. Separation is effected by projecting solids by centrifugal force against a bed of solids as the gas phase makes a 180.degree. directional change. The solids phase, however is required to undergo two 90.degree. directional changes before exiting the apparatus.
Larson, U.S. Pat. No. 3,835,029 discloses a downflow catalytic cracker entering a cylindrical separator with a series of openings in the outside wall through which the hydrocarbon passes. The catalyst solids pass downwardly to a stripper section and then into a regenerator. Within the equipment and spatial constraints normally encountered in a fluidized bed environment, the separator of Larson would be relatively inefficient because there is no progressively increasing lateral flow path as a function of the height of openings to help effectuate separation once the mixed phase gas solids stream enters the separator.